Drop-stitch fabric pneumatic bags are used in a wide variety of applications. In one important application, drop-stitch fabric pneumatic bags are used to prop up and support heavy machinery, such as damaged aircraft sections.
As explained in the paper “Recent Advancements in the Development of Inflatable Multi-Hull Boats” (Daniel G. Bagnell, 11th International Conference on Fast Sea Transportation, Honolulu, Hi., September 2011), drop-stitch fabric (also commonly termed “drop-thread” fabric) can be compared to cored, composite structures, i.e., a core material with an outer laminate on both faces. The cored material in drop-stitch fabrics consists of two pieces of fabric that are connected, sandwich style, by a series of rows of “stitches,” The “stitching” process leaves an excess amount of thread between the two pieces of fabric in such a manner that when the two pieces of material are separated, the faces of the panel are held at a fixed distance from each other at a distance which is equal to the length of the excess thread.
Drop-stitch fabric is typically woven from polyethylene terephthalate or nylon thread. It is not stitched, although it appears to be stitched, hence the name “drop-stitch.” It can be found that each thread used in the weaving process is woven several times in the warp direction, then up (or down) forming the “drop stitch,” and then several times in the weft direction before running down (or up) again. This process assures that the fabric cannot unravel. The weaving process is such that there are approximately 50 “stitches” or threads per square inch. The thickness of this core can be as small as desired, hut generally does not get much less than 2 inches. Maximum thicknesses are reported to be as much as 30 inches and are a function of the physical constraints of the weaving loom. Additionally, some looms are capable of varying the length of the threads linearly in one direction to form a complex structure.
Drop-stitch fabric are typically woven in different grades, such as standard grades and heavy duty grades. The standard grade is generally rated for use up to about 0.7 bar and the heavy duty to about 1.0 bar.
Greige drop-stitch fabrics are not air-tight and must be coated with a sealant. FIGS. 1 and 2 illustrate an inflatable, pneumatic panel 10 made from a finished drop-stitch fabric 12, comprising a greige drop-stitch fabric 14 covered with a sealant 16.
The advantages of drop-stitch fabrics are two-fold. First, drop-stitch fabrics demonstrate structural properties similar to plywood. A drop-stitch panel can withstand large loads, and associated large deflections, before failure. However, unlike plywood where the plys will delaminate at failure, if a drop-stitch panel collapses under a large load, it will rebound to its original shape when the load is removed.
The second advantage of drop-stitch fabrics is that, if properly manufactured, drop-stitch fabrics can be used to form pneumatic structures which, when inflated, are not only rigid but also constructed to take on a predetermined shape. The shapes can be quite complex. For example, drop-stitch fabrics have been used to form fully air-worthy inflatable aircraft. FIG. 2 illustrates a common form of drop stitch panel wherein the shape of the panel is a rectangular parallelepiped. FIG. 3 illustrates a plurality of rectangular parallelepiped-shaped drop-stitch panels being used to support the wing 18 of an aircraft 20.
One of the challenging problems in making a drop-stitch fabric is that, in the course of processing and handling the fabric, the fabric can become misaligned. That is to say a drop-stitch fabric can be pulled in the “x” or “y” axis in ways that are not always uniform. Such misalignment of the fabric (often termed “bow” or “scalloping”) is caused by the top and/or bottom backing layers being “stretched” more at the middle than at the edges (or vice versa) during weaving, scouring, drying and/or rewinding.
It is difficult to see misalignments during production, as all the yarn woven in both warp (length) and weft (width) is of the same color. The manufacturer can easily scour and dry 200 yards of fabric and never realize that it is misaligned. Once the fabric is coated with sealant, the misalignment is essentially locked-in and cannot be easily corrected. This can be extremely costly to the manufacturer.
An example of an unacceptable drop-stitch fabric product is shown in FIGS. 4A and 4B which illustrates a rectangular parallelepiped-shaped drop-stitch fabric pneumatic bag 8 wherein the upper surface 9 is “bowed,” rather than being flat.
There is a need, therefore, for a drop-stitch fabric and a method of forming a drop-stitch fabric which avoid the aforementioned problems in the prior art.